1. Field of the Invention
The present invention relates to programmable resistance memory devices, and methods for operating such devices.
2. Description of Related Art
Many electronic systems need a nonvolatile memory having very long retention times at high temperature, and yet are also rewritable for updating codes and data stored therein. However, not many memory cell technologies can satisfy both of these requirements.
Phase change based memory materials, like chalcogenide based materials and similar materials, can be caused to change phase between an amorphous and a crystalline phase by application of electrical current at levels suitable for implementation in integrated circuits.
In conventional phase change memory, data is stored by the application of current which heats the phase change material to cause a transition of an active region between amorphous and crystalline phases. Because the phase change occurs as a direct result of heating, memory cells having phase change memory elements can suffer drift in resistance, as the active region composition shifts from amorphous to crystalline phase, or vice versa, due to environmental conditions to which the device is exposed.
For example, a phase change memory cell in which the active region has been reset to a generally amorphous state may over time develop a distribution of crystalline regions in the active region. If these crystalline regions connect to form a low resistance path through the active region, when the memory cell is read a lower resistance state will be detected and result in a data error. See, Gleixner, “Phase Change Memory Reliability”, 22nd NVSMW, 2007. Similar issues can arise in other types of programmable resistance materials.
It is therefore desirable to provide programmable resistance memory cells and methods for operating such devices which address the data retention issues discussed above.